As an astigmatism correction method in an optical system of an optical head which is used for recording information in or reproducing information from an optical disk, there has been a method to adjust a position of an optical member in the optical head according to a visual observation of ellipticity of an interference ring caused by an exiting beam from the optical head so that the ellipticity is decreased. See, for example, Japanese Patent Kokai Publication No. 61-281209 (page 3, lower-left column to upper-right column and FIG. 2).
However, because in the above-mentioned conventional correction method the adjustment of the position of the optical member is based on ellipticity of an interference ring which is difficult to be judged and is likely to be differently judged according to each of observers, the conventional method has problems that astigmatism correction accuracy is not ensured and the adjustment is difficult and time-consuming.